Field of the Invention
The present invention relates to wireless communications, and more particularly, to a communication method in a time division duplex (TDD)-based wireless communication system, and a wireless device using the method.
Related Art
3rd generation partnership project (3GPP) long term evolution (LTE) evolved from a universal mobile telecommunications system (UMTS) is introduced as the 3GPP release 8. The 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) in a downlink, and uses single carrier-frequency division multiple access (SC-FDMA) in an uplink. The 3GPP LTE employs multiple input multiple output (MIMO) having up to four antennas. In recent years, there is an ongoing discussion on 3GPP LTE-advanced (LTE-A) evolved from the 3GPP LTE.
To decrease an interference caused by uplink transmission between user equipments (UEs), it is important for a base station (BS) to maintain an uplink time alignment of the UEs. The UE may be located in any area in a cell. An uplink signal transmitted by the UE may arrive to the BS at a different time according to the location of the UE. A signal arrival time of a UE located in a cell edge is longer than a signal arrival time of a UE located in a cell center. On the contrary, the signal arrival time of the UE located in the cell center is shorter than the signal arrival time of the UE located in the cell edge.
To decrease the interference between the UEs, the BS needs to perform scheduling so that uplink signals transmitted by the UEs in the cell can be received every time within a boundary. The BS has to properly adjust transmission timing of each UE according to a situation of each UE. Such an adjustment is called an uplink time alignment. A random access process is one of processes for maintaining the uplink time alignment. The UE acquires a time alignment value (or also referred to as a timing advance (TA)) through the random access process, and maintains the uplink time alignment by applying the time alignment value.
When it is said that a UE supports a half duplex operation in time division duplex (TDD), it implies that uplink (UL) transmission and downlink (DL) reception are both possible, but only one of the UL transmission and the DL reception can be performed at one time. If the half duplex operation of the UE is guaranteed, an interference between a UL transmission signal and a DL reception signal can be avoided, and a burden of using a full duplex module which is relatively expensive can be decreased.
Recently, a plurality of serving cells are introduced to provide a higher data rate. For a reason of a frequency feature, coverage, or the like among the plurality of serving cells, the plurality of serving cells to which different time alignment values are applied may be configured. Accordingly, DL reception and UL transmission may overlap at a specific time point, which implies that the half duplex operation cannot be guaranteed for a UE.